1. Field of the Invention
This invention relates to an apparatus and method for drying board materials. More particularly, the present invention relates to an improved drying method and apparatus wherein cooler air at widely-varying and well controlled temperatures is directed to the areas of boards most susceptible to overdrying.
2. Description of the Background Art
The use of board dryers is known in the prior art. One application for board dryers is in the manufacture of gypsum wallboard. Gypsum wallboard is made by placing a gypsum slurry between two continuous paper sheets. This laminate is then shaped, cut into appropriately sized lengths, and dried. The present invention is an improved board drying apparatus and method.
In particular, the present invention is directed at overcoming problems of excessive and uneven board drying inherent in prior art. Most board dryers deliver an airstream of a uniformly high temperature over a number of boards as they travel along a conveyor. However, because boards typically are arranged on the conveyor alongside one another in an array, an uneven drying is achieved. This occurs because the wet boards on the conveyor act as a heat sink. This heat sink greatly reduces the temperature of the airstream delivered over the boards. Moreover, the effect of this heat sink is greatest for the interior boards of the array because they are surrounded by other boards. Alternatively, the effects of the heat sink are minimized for the exposed external edges in the board because there are no adjacent boards present. As a result, in prior board dryers the outside edges of the boards are exposed to higher temperature airflows and are more susceptible to overdrying, thermal damage and discoloration. The interior boards of the array, and their corresponding edges, are exposed to cooler temperatures, thereby minimizing the possibility of overdrying. The present invention is aimed at eliminating overdrying and achieving a uniform drying of board materials
The background art contains many examples of improved board dryers. For instance, U.S. Pat. No. 3,140,929 to Johanson generally teaches the use of devices for rapid and effective cooling of dried wall-board sheets for efficient stacking and storing of the finished sheets without risk of damage. In particular, dampers are used to make possible a commingling of local air into supplied cooling air for complete cooling of already dried boards.
Additionally, U.S. Pat. No. 3,435,535 to Blair discloses methods for drying gypsum boards, while not overdrying their edges. The devices used to prevent such overdrying comprise a series of ducts, and a system of superposed arrays of ducting, wherein under-walls of each duct are provided with a plurality of apertures to permit air jets to be expelled downwardly so as to impinge upon the marginal areas and around the edges of the wallboard. The effect of the multiplicity of air jets is to form side curtains as a partial barrier to the main or central hot air streams of each passageway and to form jets of air directed toward the board edges so that the hot air streams become mixed with the cool air flowing from the apertures (and the temperature of the air is reduced at the locations near the edges of the wallboard).
Still yet another example is provided by U.S. Pat. No. 2,909,850 to Loechl. This patent is directed to modifying dryer apparatuses to protect the edge portions of drying gypsum boards from being over-dried and calcined. The edge portions of the board being dried are protected from the rapid circulation of the hot drying medium. This protection allows for the maintenance of temperatures higher than normal in the drying of the gypsum board and protection for the edges of the board from over-drying with subsequent calcination. For the desired isolation of the edge portions of boards being conveyed through the dryer, there is a partition-like construction which is positioned toward each end of the conveyor roll series. This protective partition is mounted and spaced somewhat inwardly from the ends of the rolls for the purpose of protecting the board edges from calcination while being dried.
Finally, U.S. Pat. No. 3,043,014 to Loechl is directed to a process for applying at the end edges a coolant which retards the rate of evaporation of water from the end portions of the board to the extent that the end portions of the board dry at the same time as the body portion of the board. This allows for the end edge portions, not being dried prior to the drying of the main body portion, resist recalcination during drying. The application of the coolant proceeds via spraying.
Although each of the above referenced inventions achieves its individual objective, they all suffer from common drawbacks. The improved drying apparatus and method of the present invention is aimed at overcoming these drawbacks.